Seismicity potential of geothermal operations in the North Sea Supergroup in the Roer Valley Graben

For the extended summary (Dutch) see: Samenvatting Ondiepe geothermie in de Roerdalslenk

The report describes an exploratory study regarding the geothermal potential of shallowly buried (<1500 m) sedimentary formations of Cenozoic age in the Netherlands, particularly in the Roer Valley Graben (RVG). These formations, part of the North Sea Supergroup, are seen as promising for geothermal heat production due to their thick and deeply buried sequences. However, the RVG is considered a high-risk area for geothermal exploitation due to natural seismicity. Prior geothermal projects targeting deeper Dinantian carbonates have been linked to seismic events up to ML 1.7, but the shallower Cenozoic sediments are expected to have a lower potential for induced seismicity due to inelastic deformation.

Energie Beheer Nederland B.V. (EBN) commissioned TNO to investigate a first order reactivation and seismogenic potential of faults in these sediments. The study included:

1. A literature review and analysis of the KNMI seismicity catalog to assess evidence of natural seismicity in the North Sea Groups.
2. An inventory of reservoir, rock, and fault properties, in-situ stresses, and pressures to generate input for geomechanical modeling and identify data gaps.
3. A model study of fault reactivation using the SRIMA tool, assuming linear elasticity.
4. Exploratory models including elasto-plastic reservoir behavior using a Finite Element model.

The study found that fault reactivation does not necessarily lead to seismic slip. For faults to become seismogenic, reactivation and specific fault friction behavior are required. The literature review indicated limited evidence of shallow seismic activity in the North Sea Supergroup, with most seismicity confined to deeper formations. Data inventory revealed a lack of reliable data for the North Sea Group layers due to limited and poor-quality data collection. Therefore, to understand the impact of expected geological variations, four generic but realistic case study locations were selected for simulations, aiming to represent different thicknesses, fault patterns and depth location of the sediments in the RVG system.

SRIMA calculations showed a substantial probability (40-80%) of fault reactivation near the injection well, decreasing with distance from the well. Including elasto-plastic behavior and more realistic reservoir behavior reduced the probability of reactivation. The study recommends further investigation of the seismogenic potential through follow-up modeling, laboratory studies, and local monitoring. It also suggests improvements to SRIMA or alternative methods to better represent material behavior and stress build-up in the poorly consolidated reservoirs.